Recently, with the recent rapid increase in oil price, much attention has been paid to gas to liquid (GTL) processes using natural gases instead of petroleum as fuels for transportation or raw materials in the petrochemical industry. In fact, the researches on Fischer-Tropsch synthesis, a carbon monoxide hydrogenation, had not been active until early 1970s, but with the recent increase in oil price, Fischer-Tropsch method is gaining again much attention.
Of the techniques involved in the GTL process, which have various techniques including those for reforming and purifying synthetic gases, the Fischer-Tropsch synthesis reaction, by which synthetic hydrocarbon is synthesized from a synthetic gas, is considered a core technique.
Various products may be synthesized according to the composition of synthetic gas and a catalyst used in the Fischer-Tropsch synthesis process to produce synthetic oil from synthetic gas (CO+H2).
In general, if a synthetic gas having a H2/CO ratio of 2 or higher is used for the Fischer-Tropsch synthesis process, a large amount of heavy hydrocarbon products is synthesized. On the other hand, if synthetic gas having a H2/CO ratio of less than 2 is used for the Fischer-Tropsch synthesis process, gasoline (C5-C11), diesel (C12-C18), wax (>C24), or the like are synthesized.
Fe-based and Co-based Fischer-Tropsch catalysts are used in an industrial scale according to the H2/CO ratio.
Further, various chemical products such as hydrocarbon, alcohol, ether, and acetic acid can be synthesized by varying synthetic conditions.
U.S. Pat. No. 5,543,437A (Charles B. Benham, Arvada) and WO 2005/090521 (CompactGTL plc, Mike Bowe, Joseph) disclose a process for the production of long-chain hydrocarbons, such as synthetic oil, from natural gas.
In general, reactors for Fischer-Tropsch synthesis can be classified into fixed bed reactors (FBR), slurry bubble column reactors (SBCR), and fuidized bed reactors. At present, the fixed bed reactors and the slurry bubble column reactors are widely used.
The slurry bubble column reactor is more advantageous than the fixed bed reactor as a pilot-scale reactor for Fischer-Tropsch synthesis as follows.
The slurry bubble column reactor:
1) has high efficiency of heat transfer,
2) has no pressure drop and no temperature gradient along an axial direction of the reactor (i.e., no hot spot),
3) can add, discharge, and restore a catalyst during the operation,
4) can be easily installed,
5) can be installed in a cost-effective manner,
6) has high yield (the amount of products per reactor volume), and
7) has a large capacity of reactor.
Based on these advantages, slurry bubble column reactors are more widely used than the fixed bed reactors. However, slurry bubble column reactors require a slurry recirculation device and a separator which separates solid catalysts and liquid products from the slurry.
In addition, since catalyst particles in the slurry bubble column reactor are attrited to finer particles with a lapse of time during the operation, efficiencies of the slurry recirculation device and the separation device are decreased so that the catalyst particles may be discharged out of the slurry bubble column reactor. Thus, products cannot be uniformly and continuously obtained since the concentration of the slurry and operation conditions in the slurry bubble column reactor are changed.
U.S. Pat. No. 5,599,849A (Berend Jager) discloses a back flush process upon the slurry separator in a slurry bubble column reactor for Fischer-Tropsch synthesis. Liquid products are continuously separated and discharged using a plurality of filtering medium units, as a slurry separation device, installed at an upper portion of the slurry bubble column reactor. When a pressure drop of equal to or greater than 8 bar is detected in the slurry separation device, the slurry separation device is restored to an initial state using a back flush of liquid products and high pressure gases.
However, according to the above patent, pressure loss applied to the slurry separation device is increased with a lapse of time during the operation. Accordingly, pressure in the reactor is increased to increase the level of reactants, and thus the concentration of the slurry is decreased. Theses change in operation conditions cannot induce a uniform Fischer-Tropsch synthesis reaction. Furthermore, the products and the high pressure gases, which are media of the back flush process, further increase the pressure in the reactor, thereby interfering with regular operations.
Further, a short-chain hydrocarbon instead of the desired long-chain hydrocarbon may be obtained since the separation device and the product discharge device are disposed at an upper portion of the reactor since the desired products of the Fischer-Tropsch reaction may be positioned at the lower portion of the reactor due to increased viscosity and specific gravity with the growth of the chain.
U.S. Pat. No. 5,422,375A (Erling Rytter) discloses a method of continuously separating and discharging slurry in a reactor for Fischer-Tropsch synthesis by installing a slurry separation device for pressure fluctuations in the reactor and sensing a vacuum state in the slurry separation device when the level of reactants is increased.
However, the method is not practical, and has a disadvantage that the separation device cannot be easily repaired once it becomes defective.
U.S. Pat. No. 7,144,924 B2 (Gabriele Carlo Ettore Clerici) discloses a hydro-cyclone for separating slurry. The efficiency of the hydro-cyclone is significantly influenced by the concentration of the slurry and catalyst particle size distribution.
Unfortunately, however, since catalyst particles in the slurry bubble column reactor are abraded with a lapse of time during the operation, the hydro-cyclone may not be practical.
Thus, there is a need to develop a Fischer-Tropsch synthesis method by which solid catalyst and product mixture slurry is continuously separated and high-quality long-chain hydrocarbon products are uniformly discharged by the amount of the synthesized products in the slurry bubble column reactor for Fischer-Tropsch synthesis.
[Reference No. 1] U.S. Pat. No. 5,543,437 A (Charles B. Benham, Arvada) 1996 Aug. 6
[Reference No. 2] WO 2005/090521 (CompactGTL plc, Mike Bowe, Joseph) 2005 Sep. 29
[Reference No. 3] U.S. Pat. No. 5,599,849 A (Berend Jager) 1997 Feb. 4
[Reference No. 4] U.S. Pat. No. 5,422,375 A (Erling Rytter) 1995 Jun. 6
[Reference No. 5] U.S. Pat. No. 7,144,924 B2 (Gabriele Carlo Ettore Clerici) 2006 Dec. 6